1. Field of the Invention
The present invention relates generally to electronic circuits and, in particular, to an adaptive charge circuit which provides an initial current pulse to turn on a power FET and then provides low, adaptive holding current to maintain the gate of the power FET at the "on" potential with minimum current drain from the charge supply.
2. Discussion of the Prior Art
The gate impedance of a large power MOSFET transistor is highly capacitive. Hence, the terms "charge" and "discharge" are used to describe turning the power transistor on and off, respectively.
In all high-side driver applications (and in low-side drivers operating from low supply voltages), the gate of the power FET must be raised above the supply potential by gate charge circuitry in order to achieve a low Rds (on) in the FET. This requires the use of a charge pump to supply the gate charge circuitry. Since charge pumps typically have very high output resistances, design of the gate charge circuit must minimize extraneous current drain from the charge pump. That is, current which is flowing from the charge pump but is not flowing into the gate of the power FET should be minimized.
Typically, low current drain is achieved by utilizing MOS charge circuits, since MOS devices can act as true on/off switches.
U.S. patent application Ser. No. 07/353,123, filed May 17, 1989, by Stephen W. Hobrecht and commonly-assigned herewith, discloses a charging circuit for a diffused metal oxide semiconductor transistor (DMOST) power device. The Hobrecht charging circuit, which utilizes a combination of bipolar devices and MOS switches, employs a differential amplifier gate driver that operates at a relatively low quiescent current when the DMOST is off. When the DMOST is to be switched from off to on, the differential amplifier tail current is briefly raised to a substantially higher value so that the DMOST parasitic gate capacitance can quickly be driven to the on level. While the Hobrecht charging circuit provides definite advantages over previously known charging circuits, a major deficiency of MOS switches is that, in some harsh operations environments, the breakdown voltages of the MOS transistors are insufficient for the transistors to withstand high voltage transients. For example, in automotive systems, the possibility of inadvertent reverse battery conditions or loose battery cables has caused some automobile manufacturers to specify that integrated circuits utilized in these applications be capable of withstanding up to 60 V, well above the breakdown voltage of commonly available MOS transistors. Thus, additional protective circuitry is required to shield the MOS switching transistors from voltage transients. This additional circuitry not only makes the product more expensive but also utilizes IC die area with an accompanying decrease in yield.
Another deficiency of conventional charging circuitry is the use of voltage drive at the power FET gate in order to obtain high speed on/off switching. Given that ##EQU1## it can easily be seen that rapid changes in voltage cause high current flow. This high current flow creates an electric field which may be disruptive to other circuitry which is located in close physical proximity to the charging circuitry. For example, in automotive applications, the electric field caused by a voltage drive charging circuit may affect on-board computers that control braking and/or acceleration.